The invention concerns a combination of the press section and the initial part of a multi-cylinder drying section in a paper making machine, said press section comprising a compact press roll arrangement, whose rolls form press nips with one another, and said press section further comprising a separate press, whose press rolls form a press nip, and the drying section in said combination comprising a multi-cylinder dryer consisting of different cylinder groups.
As is well known, in paper or paperboard making machines press sections are commonly used which comprise a compact press roll combination followed by a separate press nip. Examples of compact press sections include the applicant's "Sym-Press II" (.TM.) and "Sym-Press O" (.TM.) press sections, in connection with which it is possible to use a separate press nip, or if such a nip is used, in the paper machine space usually can be reserved for a separate nip so that such a nip can be introduced later relatively easily.
In the prior art press sections, problems have occurred in the transfer of the web from the center roll of the compact press roll to the separate press. As is well known, this transfer has taken place by means of a paper guide roll fitted in proximity to the center roll in the press and by means of a suction roll placed underneath said paper guide roll, the lower felt in the separate nip running over said suction roll. On the substantially horizontal run of the lower felt, even though means of negative pressure have been arranged inside the loop of the felt, detaching of the web from the lower felt as a result of blowing has occurred, which has caused web breaks. These problems concerning web draw have been aggravated with increasing running speeds of paper machines.
Moreover, in the prior art draws between a compact press section and a separate press, a problem existed in the handling of broke, this problem arising primarily from congested space. For the purposes of this application "broke" can be defined as partly or completely manufactured paper or paperboard that does not leave the machine as saleable paper or paperboard. In said prior-art draws, a separate broke conveyor has been necessary, and for the conveyor an opening must be provided in the frames of the machine at the operation side. It is difficult to place such a conveyor in a congested space.
In an arrangement known in the prior art, the separate nip is supported on frame parts, which are directly connected to the rest of the frame part of the press section. Thus, in the separate press, some of the same frame parts are employed as in the preceding compact press section. This frame arrangement has resulted in a problem wherein oscillations are transferred from frame parts to other frame parts. These problems are aggravated by the fact that, owing to the small space that is available, the various frame parts cannot always be constructed to be sufficiently rigid to withstand the occurrence of these vibrations.
Attempts have been made to control such draw and blow problems by means of suction rolls, suction boxes, and various doctor arrangements as well as by, before the nip of the separate press, providing a certain covering angle which reduces the vibration of the web before the separate nip. However, by means of these arrangements, it is not possible to control said problems completely, because, for example, when suction rolls and suction boxes are used, the suction effect becomes weaker when the permeability of the lower felt in the separate nip is increased during operation.
It is known in the prior art to employ drying of the web between separate nips. In this respect, reference is made to the Applicant's FI Pat. Appl. 763761, to the SE Patent 97,609, and to the book by Gunnar Gavelin, Fourdrinier Papermaking 1963, page 125, FIG. 5, "Arrangement for a suction hot press". At the time of the above cited papers, the running speeds of paper machines were, however, so relatively low that the problems discussed above and related to the draw of the paper web did not occur. Nor were compact press sections in connection with a separate press known at that time.